Generally, X-ray tubes are employed in medical diagnostic systems, industrial diagnostic systems and the like. X-ray tubes are used for checking foreign matters or analyzing materials in the industrial field and the like. X-ray analysis is an analysis on components of various types of materials or composition of products. An X-ray tube employed in X-ray analysis comprises an anode, a cathode and an envelope. Further, in a general type of X-ray tube, a beryllium (Be) window is used as its X-ray transmission window. The Be window is a part of the envelope and transmits an X-ray beam to be used (or releases a beam to the outside).
The above-described X-ray tube for analysis has the following problems. That is, when the X-ray tube is continuously used while an outer surface of the Be window exposed to the atmosphere, the Be window itself and the brazed portion between the Be window and the envelope are corroded while in use, thereby frequently destroying the vacuum state of the envelope. Note that the Be window needs to be thinner in order to improve the accuracy of analysis, but the above-described problems, in particular, are exacerbated.
The following (1 and 2) are known as techniques for suppressing such problems.
(1) A polyimide resin-forming liquid is applied onto the outer surface of the Be window, followed by drying and baking, and thus a polyimide resin coating which is hardly deteriorated to X-rays is formed on the outer surface of the Be window.
(2) Besides resins, an inorganic coating such as of a boron compound is used as a protecting film.